Heat-treatable beryllium-copper alloys have been known for several decades and are noted for having good formability and machinability. Metallurgists have sought, by various heat treatments, to provide such alloys with high mechanical strength while retaining high electrical conductivity. In the past, the strengths of these alloys were increased by cold working and age hardening. However, it was found that if the alloy is allowed to recrystallize during aging, the cold worked structure and the associated increased strength are lost. In order to age the alloy below the recrystallization temperature, small quantities of zirconium and/or hafnium were introduced into the alloy to raise the recrystallization temperature of the alloy after cold working.
Beryllium conductor bronzes are high conductivity, wrought and casting beryllium-copper based alloys containing between about 0.25 and 0.7% by weight beryllium, up to about 2.8% cobalt, and up to about 2.0% nickel. These alloys, which are discussed in detail by Pfeiffer and Honig in Metallwissenschaft und Technik, vol. 22(11), pp. 1125-9 (November 1968), have electrical conductivities of up to 32 m/ohm mm.sup.2 and good tensile strengths (up to 140 ksi) at elevated temperatures. By subjecting heat-treatable beryllium conductor bronzes to two separate heat treatments, it was found that one could optimize these desirable properties. The alloy is first subjected to an annealing or solution heat treatment which consists of heating the alloy to solutionize the beryllium in the copper matrix. The beryllium is then "frozen" in the solution by rapidly quenching the alloy to room temperature. Cold working followed by age hardening optimizes the hardness and tensile strength properties. During the age hardening step, the electrical conductivity of the alloy is found to increase with time.
The Lane et al. U.S. Pat. No. 3,196,006 teaches a beryllium-copper based alloy containing 0.4 to 0.7% by weight beryllium, 2 to 3% cobalt and/or nickel, 0.12 to 0.4% zirconium and/or hafnium and the balance copper with small amounts (&lt;0.05%) of incidental impurities (including lead). These alloys are disclosed for use in the fabrication of large electrical generator components such as rotor wedges which can be cast to shape or wrought by hot and cold working.
Rotor wedges secure the copper windings of the rotors which rotate at high velocities. Under normal operating conditions the periphery of these rotors travel at almost sonic speeds. The centrifugal force generated by these conditions exerts as much as 25,000 psi (1800 kg/cm.sup.2) stress on the wedges. These stresses occur at operating temperatures as high as 200.degree. C. Unusual circumstances can cause the rotor to occasionally operate at even higher speeds and temperatures, adding still more stress and heat to the rotor wedges. According to the Lane et al. patent, as-cast or wrought beryllium-copper for use in making rotor wedges is treated by a process which involves annealing at temperatures of about 750.degree. C. and above, followed promptly by quenching. Next, the members are cold worked and then aged at temperatures of from 300.degree.-600.degree. C. to develop desirable strength and hardness.
Copper Alloy CA 175 is a commercial alloy containing nominally 2.5% cobalt and 0.5% beryllium. When CA 175 undergoes precipitation hardening treatment, it acquires a highly desirable combination of strength (100-120 ksi) and high electrical conductivity (greater than 45% that of pure copper) at room temperature. Due to these properties, the alloy is useful for making resistance welding electrodes, electrical switches, springs, contacts, or tooling material. These desirable properties, however, are temperature dependent; at elevated temperatures, i.e., above 100.degree. C., CA 175 becomes notch sensitive when exposed to long term constant stress loading. Under these conditions CA 175 exhibits low fracture toughness. Impact or cyclic loading conditions at elevated temperatures also result in low fracture toughness. Notch sensitivity and resultant low fracture toughness can lead to serious premature failure in parts having notched designs. This tendency for notched parts to prematurely fail at elevated temperatures in a brittle mode has excluded CA 175 from those applications, e.g., rotor wedges, which require a combination of high strength and electrical conductivity at elevated temperatures.
As previously mentioned, generator rotors are subjected to high stress from the centrifugal forces present during the generation of electricity. Furthermore, these generators operate at temperatures as high as 400.degree. F. (204.degree. C.). Due to these high temperature stress conditions, CA 175 processed according to the prior art has a tendency to fail prematurely in notched stress rupture at operating temperatures as low as 302.degree. F. (150.degree. C.). Even the copper-based alloys disclosed in the aforementioned Lane et al. patent have demonstrated poor stress rupture properties when evaluated under conditions similar to those for the CA 175 determinations.
A need has therefore existed, particularly in the electrical generator field, for shaped beryllium-copper alloys having improved high temperature strength and especially notched stress rupture resistance, while retaining good electrical and thermal conductance properties.
Accordingly, it is an object of the present invention to provide copper-based alloys having a combination of high strength and conductivity.
Another object is to provide shaped beryllium-copper alloys which retain optimum tensile strength and electrical conductivity at high use temperatures.
Another object is to provide improved shaped beryllium-copper alloys useful in fabricating rotor wedges for electrical generators and which retain optimum notched stress rupture resistance and thermo-electrical conductivity at high operating speeds and temperatures.
Another object is to provide a process for treating copper-based alloys to impart thereto a desirable combination of high strength and conductivity.
Another object is to provide a process for thermally treating shaped beryllium-copper alloys to impart optimum strength and electrical conductivity retention at high use temperatures.
Yet another object is to provide an improved thermal treatment for shaped beryllium-copper alloys which are used in fabricating rotor wedges for electrical generators and which retain optimum notched stress rupture resistance and thermo-electrical conductivity at high operating speeds and temperatures.
These and other objects of the invention as well as an understanding of the advantages thereof can be had by reference to the following disclosure and claims.